Lithographic apparatus and method

ABSTRACT

A lithographic apparatus includes a radiation source configured to produce a radiation beam, and a support configured to support a patterning device. The patterning device is configured to impart the radiation beam with a pattern to form a patterned radiation beam. A chamber is located between the radiation source and patterning device. The chamber contains at least one optical component configured to reflect the radiation beam, and is configured to permit radiation from the radiation source to pass therethrough. A membrane is configured to permit the passage of the radiation beam, and to prevent the passage of contamination particles through the membrane. A particle trapping structure is configured to permit gas to flow along an indirect path from inside the chamber to outside the chamber. The indirect path is configured to substantially prevent the passage of contamination particles from inside the chamber to outside the chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of InternationalPatent Application No. PCT/EP2011/054057, filed Mar. 17, 2011, whichclaims the benefit of priority from US provisional application61/358,645, filed on Jun. 25, 2010, and US provisional application61/362,981, filed on Jul. 9, 2010. Both of these provisionalapplications are hereby incorporated in their entireties by reference.

FIELD

The present invention relates to a lithographic apparatus and method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{NA}}} & (1)\end{matrix}$where λ is the wavelength of the radiation used, NA is the numericalaperture of the projection system used to print the pattern, k₁, is aprocess dependent adjustment factor, also called the Rayleigh constant,and CD is the feature size (or critical dimension) of the printedfeature. It follows from equation (1) that reduction of the minimumprintable size of features can be obtained in three ways: by shorteningthe exposure wavelength λ, by increasing the numerical aperture NA or bydecreasing the value of k₁.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. EUV radiation is electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm, for example within the range of 5-10 nmsuch as 6.7 nm or 6.8 nm. Possible sources include, for example,laser-produced plasma sources, discharge plasma sources, or sourcesbased on synchrotron radiation provided by an electron storage ring.

EUV radiation may be produced using a plasma. A radiation system forproducing EUV radiation may include a laser for exciting a fuel toprovide the plasma, and a source collector module for containing theplasma. The plasma may be created, for example, by directing a laserbeam at a fuel, such as particles of a suitable material (e.g. tin(Sn)), or a stream of a suitable gas or vapor, such as Xe gas or Livapor. The resulting plasma emits output radiation, e.g., EUV radiation,which is collected using a radiation collector. The radiation collectormay be a mirrored normal incidence radiation collector, which receivesthe radiation and focuses the radiation into a beam. The sourcecollector module may include an enclosing structure or chamber arrangedto provide a vacuum environment to support the plasma. Such a radiationsystem is typically termed a laser produced plasma (LPP) source.

In an alternative arrangement a radiation system for producing EUVradiation may use an electrical discharge to generate the plasma. Theelectrical discharge passes into a gas or vapor such as Xe gas, Li vaporor Sn vapor, generating a very hot plasma which emits EUV radiation.Such a radiation system is typically termed a discharge produced plasma(DPP) source.

Plasma creation within an EUV source may cause contamination particlesto be created from the fuel. These contamination particles may moveeither at relatively fast speeds, in which case they tend to generallyfollow the path of the radiation beam; or at relatively slow speeds, inwhich case they are free to undergo Brownian motion. In somelithographic apparatus the relatively slow moving contaminationparticles may be conveyed by a flow of gas within the lithographicapparatus.

Both the relatively fast moving and relatively slow moving contaminationparticles may move towards the patterning device of the lithographicapparatus. If the contamination particles reach the patterning device(even in very small numbers) then they may contaminate the patterningdevice. The contamination of the patterning device may reduce theimaging performance of the lithographic apparatus and may in moreserious cases require the patterning device to be replaced. Thepatterning device can be expensive and therefore any reduction in thefrequency with which it must be replaced may be advantageous.Furthermore, replacement of the patterning device is a time consumingprocess, during which the operation of the lithographic apparatus mayhave to be stopped. Stopping the operation of the lithographic apparatusmay reduce the output of the lithographic apparatus and thereby reduceits efficiency, which is undesirable.

It is desirable to provide a lithographic apparatus which can captureboth fast and slow moving contamination particles such that they areless likely to contaminate the patterning device.

According to an aspect of the invention, there is provided alithographic apparatus that includes a radiation source configured toproduce a radiation beam, and a support configured to support apatterning device. The patterning device is configured to impart theradiation beam with a pattern to form a patterned radiation beam. Achamber is located between the radiation source and the support. Thechamber contains at least one optical component configured to reflectthe radiation beam. The chamber is configured to permit radiation fromthe radiation source to pass therethrough. A membrane defines part ofthe chamber. The membrane is configured to permit the passage of theradiation beam through the membrane, and to prevent the passage ofcontamination particles through the membrane. A particle trappingstructure is configured to permit gas to flow along an indirect pathfrom inside the chamber to outside the chamber, the indirect path of theparticle trapping structure being configured to substantially preventthe passage of contamination particles from inside the chamber tooutside the chamber.

According to an aspect of the invention, there is provided alithographic method that includes generating a radiation beam, anddirecting the radiation beam through a chamber containing at least oneoptical component that reflects the radiation beam. The radiation beamis directed towards a patterning device. The chamber includes amembrane. The method includes preventing the passage of contaminationparticles with the membrane when the radiation beam passes from thechamber through the membrane and towards the patterning device, flowinggas from inside the chamber to outside the chamber along an indirectpath through a particle trapping structure, the indirect pathsubstantially preventing the passage of contamination particles frominside the chamber to outside the chamber, imparting the radiation beamwith a pattern to form a patterned radiation beam with the patterningdevice, and projecting the patterned beam of radiation onto a substratewith a projection system.

According to an aspect of the invention there is provided a lithographicapparatus comprising a radiation source configured to produce aradiation beam and a support configured to support a patterning device,the patterning device being configured to impart the radiation beam witha pattern to form a patterned radiation beam, wherein the support isprovided with a pellicle which comprises a layer of graphene.

According to an aspect of the invention there is provided a spectralpurity filter comprising a grid configured to prevent or reduce thepassage of infrared radiation, wherein the grid is covered with graphenewhich prevents the passage of oxygen to the grid. The graphene may beprovided as one or more layers, or may surround ribs of the grid.

According to an aspect of the invention there is provided a spectralpurity filter comprising a grid configured to prevent or reduce thepassage of infrared radiation, the grid comprising a tungsten/graphenemulti-layered structure.

According to an aspect of the invention there is provided a spectralpurity filter comprising a material which blocks out-of-band radiation,wherein the spectral purity filter further comprises a layer of graphenewhich supports the material.

According to an aspect of the invention there is provided a multi-layermirror comprising alternating layers of a first material and a secondmaterial, wherein graphene is provided between the alternating layers.

According to an aspect of the invention there is provided a multi-layermirror comprising alternating layers of a first material and a secondmaterial, wherein a layer of graphene is provided as an outer layer ofthe multi-layer mirror.

According to an aspect of the invention there is provided a lithographicapparatus having a graphene membrane which is configured to stop thepassage of contamination particles and to transmit EUV radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 is a view of an LPP source collector module SO of the apparatusof FIG. 1;

FIG. 3 is view of a membrane and particle trapping structure accordingto an embodiment of the present invention;

FIG. 4 is a cross sectional view through an embodiment of a reticlewhich may form part of a lithographic apparatus according to theinvention;

FIG. 5 is a cross sectional view through an embodiment of a reticlewhich may form part of a lithographic apparatus according to theinvention;

FIG. 6 is a cross sectional view through an embodiment of a reticlewhich may form part of a lithographic apparatus according to theinvention;

FIG. 7 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 8 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 9 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 10 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 11 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 12 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 13 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention;

FIG. 14 is a cross sectional view through an embodiment of a spectralpurity filter which may form part of a lithographic apparatus accordingto the invention; and

FIG. 15 is a cross sectional view through an embodiment of a multi-layermirror which may form part of a lithographic apparatus according to theinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 100 including asource collector module SO according to one embodiment of the invention.The apparatus comprises: an illumination system (illuminator) ILconfigured to condition a radiation beam B (e.g. EUV radiation); asupport structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask or a reticle) MA and connected to a firstpositioner PM configured to accurately position the patterning device; asubstrate table (e.g. a wafer table) WT constructed to hold a substrate(e.g. a resist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate; and a projection system(e.g. a reflective projection system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g. comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallminors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The projection system, like the illumination system, may include varioustypes of optical components, such as refractive, reflective, magnetic,electromagnetic, electrostatic or other types of optical components, orany combination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of a vacuum. It may bedesired to use a vacuum for EUV radiation since gases may absorb toomuch radiation. A vacuum environment may therefore be provided to thewhole beam path with the aid of a vacuum wall and vacuum pumps.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives an extreme ultraviolet(EUV) radiation beam from the source collector module SO. Methods toproduce EUV radiation include, but are not necessarily limited to,converting a material into a plasma state that has at least one element,e.g. xenon (Xe), lithium (Li), tin (Sn), gadolinium (Gd) or terbium (Tb)coupled with one or more emission lines in the EUV range (the EUV rangeis considered to include wavelengths around 13 nm and around 6-7 nm). Inone such method, often termed laser produced plasma (“LPP”) the requiredplasma can be produced by irradiating a fuel with a laser beam. The fuelmay for example be a droplet, stream or cluster of material having therequired line-emitting element. The source collector module SO may bepart of an EUV radiation system including a laser, not shown in FIG. 1,for providing the laser beam which excites the fuel. The resultingplasma emits output radiation, e.g. EUV radiation, which is collectedusing a radiation collector located in the source collector module. Thelaser and the source collector module may be separate entities, forexample when a CO₂, laser is used to provide the laser beam for fuelexcitation. In such cases, the laser is not considered to form part ofthe lithographic apparatus, and the radiation beam is passed from thelaser to the source collector module with the aid of a beam deliverysystem comprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thesource collector module, for example when the source is a dischargeproduced plasma EUV generator, often termed as a DPP source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g. mask)MA, which is held on the support structure (e.g. mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor PS2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam B.Patterning device (e.g. mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the support structure (e.g. mask table) MT and the    substrate table WT are kept essentially stationary, while an entire    pattern imparted to the radiation beam is projected onto a target    portion C at one time (i.e. a single static exposure). The substrate    table WT is then shifted in the X and/or Y direction so that a    different target portion C can be exposed.-   2. In scan mode, the support structure (e.g. mask table) MT and the    substrate table WT are scanned synchronously while a pattern    imparted to the radiation beam is projected onto a target portion C    (i.e. a single dynamic exposure). The velocity and direction of the    substrate table WT relative to the support structure (e.g. mask    table) MT may be determined by the (de-)magnification and image    reversal characteristics of the projection system PS.-   3. In another mode, the support structure (e.g. mask table) MT is    kept essentially stationary holding a programmable patterning    device, and the substrate table WT is moved or scanned while a    pattern imparted to the radiation beam is projected onto a target    portion C. In this mode, generally a pulsed radiation source is    employed and the programmable patterning device is updated as    required after each movement of the substrate table WT or in between    successive radiation pulses during a scan. This mode of operation    can be readily applied to maskless lithography that utilizes    programmable patterning device, such as a programmable mirror array    of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows the apparatus 100 in more detail, including the sourcecollector module SO, the illumination system IL, and the projectionsystem PS. The source collector module SO is constructed and arrangedsuch that a vacuum environment can be maintained in an enclosingstructure 220 of the source collector module SO.

A laser LA is arranged to deposit laser energy via a laser beam 205 intoa fuel, such as xenon (Xe), tin (Sn) or lithium (Li) which is providedfrom a fuel supply 200, thereby creating a highly ionized plasma 210with electron temperatures of several 10's of eV. The energeticradiation generated during de-excitation and recombination of these ionsis emitted from the plasma, collected and focussed by a near normalincidence collector optic CO.

Radiation that is reflected by the collector optic CO is focused in avirtual source point IF. The virtual source point IF is commonlyreferred to as the intermediate focus, and the source collector moduleSO is arranged such that the intermediate focus IF is located at or nearan opening 221 in the enclosing structure 220. The virtual source pointIF is an image of the radiation emitting plasma 210.

Although the source collector module SO shown in FIG. 2 comprises an LPPsource, the source collector module may comprise any suitable source andmay for example comprise a DPP source. The DPP source may for example beconfigured to pass an electrical discharge into a gas or vapor such asXe gas, Li vapor or Sn vapor, generating a very hot plasma which emitsEUV radiation. A collector optic such as a grazing incidence collectormay be configured to collect the EUV radiation and focus it to anintermediate focus. The intermediate focus may be located at or near anopening in an enclosing structure of the source collector module.

After passing through the intermediate focus IF the radiation traversesthe illumination system IL. The illumination system IL may include afacetted field mirror device 22 and a facetted pupil mirror device 24arranged to provide a desired angular distribution of the radiation beam21 at the patterning device MA, as well as a desired uniformity ofradiation intensity at the patterning device MA. Upon reflection of thebeam of radiation 21 at the patterning device MA, a patterned beam 26 isformed and the patterned beam 26 is imaged by the projection system PSvia reflective elements 28, 30 onto a substrate W held by the substratetable WT.

More elements than shown may generally be present in the illuminationsystem IL and projection system PS. Further, there may be more mirrorspresent than those shown in the Figures, for example there may be 1-6additional reflective elements present in the projection system PS thanshown in FIG. 2.

The creation of ionized plasma from fuel not only produces radiation,but also produces unwanted contamination particles. In the case wheretin (Sn) is used as fuel, these contamination particles may be producedat a rate of approximately 1000 per second. The contamination particlesmay have a size of up to about 150 nanometres, and may have a size of upto about 500 nanometres. The contamination particles may have a speed ofup to about 100 meters per second, and may have a speed of up to about1000 meters per second.

Contamination particles produced with different speeds may takedifferent paths from the plasma 210. For example, relatively fastcontamination particles may travel in the same direction as the beam ofradiation produced by the source collector module SO. Furthermore, somerelatively fast contamination particles may strike the collector opticCO and bounce off the collector optic CO such that the particles willalso follow the path of the beam of radiation. When the relatively fastmoving contamination particles follow the path of the beam of radiation,the contamination particles may bounce off the mirror devices 22, 24within the illumination system IL so that they reach the patterningdevice MA.

Relatively slow moving contamination particles may undergo Brownianmotion and hence drift through the low pressure environment of thesource collector module SO and illuminator module IL towards thepatterning device MA. Furthermore, in some lithographic apparatus, suchas that shown in FIG. 2, the illuminator module IL and/or sourcecollector module SO may comprise a gas flow conduit 32. The gas flowconduit 32 may have a gas pumped through it so as to reduce molecularcontamination within the illumination module IL. Molecular contaminationmay be the accumulation of molecules (or products of the dissociation ofthese molecules caused by the radiation beam) on the surfaces ofreflectors (or other optical components) within the lithographicapparatus. The molecules may originate from within the lithographicapparatus itself. For example, the molecules may originate from thecomponents of the lithographic apparatus, from lubricants used withinthe lithographic apparatus or from electronic systems within thelithographic apparatus. The gas pumped through the gas conduit 32 may beatomic hydrogen. In some embodiments, the gas may be pumped into the gasconduit 32 so that the gas travels in a direction towards the patterningdevice MA. In this case, the movement of the gas through the gas conduit32 may carry relatively slow contamination particles with it towards thepatterning device MA.

In some lithographic apparatus, if contamination particles reach thepatterning device MA (even in small numbers which are less than 1particle per hour), then this may have a detrimental effect on theimaging performance of the lithographic apparatus. If the patterningdevice MA becomes contaminated with contamination particles, it may benecessary to replace or clean the patterning device MA. In order toreplace or clean the patterning device MA it may be necessary to stopthe operation of the lithographic apparatus. Any downtime of thelithographic apparatus would result in a decrease in the output of thelithographic apparatus and hence reduce the profitability of thelithographic apparatus. It will be appreciated that any reduction in theprofitability of the lithographic apparatus may be undesirable.

FIG. 3 shows a schematic cross section through a portion of thelithographic apparatus indicated by 34 in FIG. 2. The portion of thelithographic apparatus shown in FIG. 3 is capable of preventing bothfast moving and slow moving contamination particles from reaching thepatterning device MA. The portion of the lithographic apparatus shown inFIG. 3 comprises first and second wall members 36, 38 which define partof the gas conduit 32. The first and second wall members 36, 38 compriserespective openings 40, 42. The openings 40, 42 share a common axiswhich is the optical axis OA of the beam of radiation 21 of thelithographic apparatus. The opening 40 in the first wall member 36comprises a gas-tight membrane 44 which is secured across the opening 40so as to prevent the passage of gas from one side of the first wallmember 36 to the other side of the first wall member 36 via the opening40.

The first and second wall members 36, 38 are spaced from one anothersuch that there is at least one gas flow path between them. In theembodiment shown in FIG. 3 there are two gas flow paths 46, one eitherside of the optical axis OA. The gas flow paths 46 allow gas to flowfrom inside the gas conduit 32 to outside the gas conduit 32 (outsidethe gas conduit 32 is indicated generally by 48). The path of gas frominside the gas conduit 32 to outside 48 the gas conduit is indicatedgenerally by the arrows 50. The gas flow paths 46 are defined byparticle trapping structures 52. The particle trapping structures 52comprise a plurality of plates 52 a. The plates 52 a extend in adirection parallel to the optical axis OA and extend alternately fromthe first wall member 36 and the second wall member 38. The plates 52 aare spaced from one another and extend in an interdigitated manner sothat they overlap in a direction perpendicular to the optical axis OA.The plates 52 a of the particle trapping structures 52 therefore ensurethat there is no line of sight path for gas to flow from inside the gasconduit 32 to outside 48 the gas conduit 32. The plates 52 a confine gasflow through the particle trapping structure 52 such that it follows anindirect path. In this case the indirect path is a meandering one, i.e.the path changes direction multiple times as it progresses towardsoutside 48 the gas conduit 32. For example, the path may changedirection at least four times as it progresses towards outside 48 thegas conduit 32. The changes in direction of the path may be abrupt. Inthe present embodiment, the meandering path is boustrophedonic, meaningthat the gas flows in one direction and then turns to go in the oppositedirection whilst it progresses towards outside 48 the gas conduit 32. Inan embodiment where the length of each plate 52 a (parallel to theoptical axis OA in use) is approximately 10 times the separation betweenadjacent plates 52 a, such a pair of adjacent plates may preventapproximately 90% of the contamination particles from passing betweenthe adjacent pair of plates. In some embodiments, it may be advantageousfor the particle trapping structure to reduce the level of contaminationparticles by 6 or 7 orders of magnitude. In these embodiments it may bedesirable to use at least 5 pairs of adjacent plates (i.e. 10 plates).It will be appreciated that any appropriate number of plates may beused. For example, there may be between 2 and 100 plates. In someembodiments, the use of curved plates (i.e. plates where there is noline of sight path through the gap between an adjacent pair of plates)may increase the proportion of contamination particles which areprevented from passing between the adjacent pair of plates compared toplanar plates. It follows that less pairs of curved plates may berequired (compared to planar plates) to prevent a given proportion ofcontamination particles from passing through the contamination trappingstructure.

In use, the openings 40, 42 are arranged within the lithographicapparatus so that the beam of radiation passes through the openings 40,42, including passing through the membrane 44. The material andthickness of the membrane 44 is chosen so that it permits the radiationbeam 21 to pass through it. It will be appreciated that in someembodiments of the invention the membrane 44 may not permit 100% of theincident radiation to pass through it. The thickness and material of themembrane 44 is also chosen so that contamination particles travelling atfast speeds in the direction of the radiation beam 21 can strike themembrane 44 without causing degradation of the membrane 44 to an extentthat it becomes no longer gas-tight.

The membrane 44 is also able to withstand the pressure created by thecollisions of many fast moving contamination particles with the membrane44 without the membrane 44 degrading such that it is no longergas-tight. For example, the membrane may need to withstand a pressure ofapproximately 1 GPa to 10 GPa created by a Sn particle. It is thoughtthat the rate of collisions may be of the order of about 1000 collisionsper second. The size of the fast moving contamination particles may bein the range of approximately 150 nanometers to approximately 1 μm. Theparticles may be travelling at speeds of approximately 100 meters persecond to approximately 1000 meters per second. It will be appreciatedthat there may be a variety of different sized particles colliding withthe membrane 44 at a variety of speeds. Furthermore, the rate ofcollisions of the fast moving particles of the membrane 44 occur may notbe constant with time.

A fast moving contamination particle travelling in the same direction asthe radiation beam 21 within the gas conduit 32 is indicated by 54. Fastmoving contamination particles which are moving in the same direction asthe radiation beam 21 (such as that indicated by 54) collide with themembrane 44 and either become impacted in the membrane 44 (as indicatedby 56) or rebound (not shown) from the membrane 44. Fast movingcontamination particles which rebound from the membrane 44 may loseenergy as they rebound. This loss of energy may cause fast movingcontamination particles which rebound off the membrane to become slowmoving contamination particles which continue to move within the gasconduit 32. If the fast moving contamination particles become slowmoving contamination particles, they may subsequently be trapped withinthe particle trapping structure 52.

Example membrane materials include zirconium (Zr), molybdenum (Mo),yttrium (Y), silicon (Si), rubidium (Rb), strontium (Sr), niobium (Nb),ruthenium (Ru) and carbon (C). An example of a range of thicknesses ofmembrane 44 which may be used is about 10 nanometers to about 500nanometers. For example, the membrane 44 may be about 100 nanometersthick. Another example of a material that may be used to fabricate themembrane is graphene. Depending on at least the strength of the membrane44 required to withstand collisions with fast moving contaminationparticles without degrading, a single sheet of graphene may be used.Alternatively, a graphene layer comprising a plurality of graphenesheets or a composite of graphene flakes may be used. It will beappreciated that the membrane 44 may be a single layer of a particularmaterial, or may be multiple layers. The layers may be formed fromdifferent materials.

As previously discussed, gas flows through the particle trappingstructures 52 from inside the gas conduit 32 to outside the gas conduit48. The flow of gas from inside the gas conduit 32 to outside 48 the gasconduit may carry with it slow moving contamination particles such asthose indicated by 58. Because the gas has to undertake an indirect paththrough the particle trapping structure 52, the path of the gas frominside the gas conduit 32 to outside 48 the gas conduit is longer than adirect path. The longer indirect flow path (compared to a shorter lengthdirect gas flow path) is defined by a greater surface area of wallsdefining the indirect gas flow path 46 (in this case a first and secondwall members 36, 38 and the plates 52 a). This greater surface area ofwalls defining the indirect gas flow path 46 that the gas is exposed toas it travels along the indirect flow path provides a greater surfacearea for the slow moving contamination particles within the gas tocontact. The increased surface area of the walls defining the indirectflow path increases the likelihood (compared to that of a direct flowpath) that the slow moving contamination particles within the gas willcontact the walls defining the indirect flow path. When the slow movingcontamination particles contact one of the walls defining the indirectgas flow path 46 within the particle trapping structure 52, they maycome to rest against the wall. Particles which have come to rest againstone of the walls which defines the indirect gas flow path 46 of theparticle trapping structure 52 are indicated by 60. Because the slowmoving contamination particles are captured by the walls defining theindirect gas flow path 46 of the particle trapping structure 52, the gaswhich has passed through the particle trapping structure 52 is free fromcontamination particles (this is indicated by the arrow 62).

As mentioned above, the indirect path of the particle trapping structure52 changes direction multiple times as it progresses towards outside 48the gas conduit 32. The changes in direction of the indirect flow pathof the particle trapping structure may be abrupt. Abrupt changes in thedirection of the indirect path of the particle trapping structure 52 mayincrease the likelihood that contamination particles will contact thewalls which define the indirect gas flow path 46, when the gasundertakes an abrupt change in direction due to the abrupt change indirection of the flow path. By increasing the likelihood thatcontamination particles will contact the walls which define the indirectgas flow path 46, it is more likely that slow moving contaminationparticles carried by the gas will come to rest against one of the wallswhich defines the indirect gas flow path 46 of the particle trappingstructure 52. In this way, abrupt changes in the direction of theindirect flow path may increase the likelihood that contaminationparticles are captured by the walls defining the indirect gas flow path46 of the particle trapping structure 52. It follows that abrupt changesin the direction of the indirect flow path 46 may reduce the likelihoodthat contamination particles pass to outside 48 the gas conduit.

It can be seen that the combination of a membrane formed from a materialand thickness sufficient to prevent fast moving contamination particlespassing through it (and also sufficient to prevent the membrane frombecoming non gas-tight), and a particle trapping structure configured tocollect slow moving contamination particles will result in alithographic apparatus which is capable of preventing both fast and slowmoving contamination particles from reaching the patterning device MA.

It will be appreciated that the above embodiment of the invention isdescribed by way of example only and that the scope of the inventionshould not be limited by such an example. It will further be appreciatedthat various modifications may be made to the embodiment of theinvention described above without departing from the scope of theinvention.

As previously discussed, the membrane 44 may be constructed of anysuitable material that is impermeable to fast moving and slow movingcontamination particles. In practice, this is likely to mean that themembrane 44 is gas-tight. However, it is within the scope of theinvention that the membrane may be gas permeable, provided that thecontamination particles cannot pass through the membrane. The membranematerial should be capable of withstanding multiple collisions with fastmoving contamination particles so that the ability of the membrane 44 tobe impermeable to both fast moving and slow moving contaminationparticles does not degrade. The membrane material should allow theradiation beam 21 of the lithographic apparatus to pass through it fromthe radiation source SO to the patterning device MA.

In the described embodiment, the plates 52 a of the particle trappingstructures 52 are generally planar. It will be appreciated that theplates 52 a may have any appropriate shape. For example, the plates maybe non-planar and/or curved. The plates may be constructed from anyappropriate material. It may be advantageous for the plates 52 a to beconstructed from a material which is non-reactive in the atmosphere inwhich the particle trapping structure is used. For example, if theparticle trapping structure is used in a hydrogen-rich atmosphere,suitable, non-reactive materials include aluminium (Al), tungsten (W),ruthenium (Ru), molybdenum (Mo), silicon oxide (SiO₂), zirconium oxide(ZrO₂) and aluminium oxide (Al₂O₃).

It will be appreciated that although the indirect gas flow path 46defined by the particle trapping structures 52 in the describedembodiment is meandering (specifically, boustrophedonic), anyappropriate structure which creates an indirect gas flow path may beused. For example, the path may be labyrinthine. It will further beappreciated that the particle trapping structure may take anyappropriate form. The particle trapping structure should be capable ofpermitting a flow of gas from inside a chamber (in this case the gasconduit 32) to outside the chamber, whilst at the same timesubstantially preventing the passage of contamination particles withinthe gas from inside the chamber to outside the chamber. Another exampleof a possible particle trapping structure is a sponge-like materialwhich is porous to the gas but impermeable to the contaminationparticles within the gas. Examples of suitable sponge-like materialsinclude sponge-like materials constructed from metals. It is desirablethat the material used to construct the sponge like material isnon-reactive in the atmosphere in which the particle trapping structureof which it forms part will be used. For example, if the particletrapping structure (and hence the sponge-like material) is to be used ina hydrogen atmosphere, then appropriate, non-reactive metals includeAluminium (Al), Tungsten (W), ruthenium (Ru) and molybdenum (Mo). Thesponge-like material comprises a lattice defining a plurality ofcavities. The sponge-like material may permit gas to flow from insidethe chamber to outside the chamber along an indirect path. The path ofthe gas through the sponge-like material is indirect because the gastravels in a non-straight line path through a series of adjacentcavities within the sponge-like material.

It will be appreciated that within the particle trapping structure ofthe described embodiment, the slow moving contamination particles withinthe gas become attached to the walls which define the indirect gas flowpath 46 through the particle trapping structures 52. The attachment ofthe contamination particles to the walls defining the indirect gas flowpath 46 of the particle trapping structure 52 may be caused by Van derWaals' forces between the contamination particles and the wall members.In some embodiments it may be desirable to coat the particle trappingstructure with a substance which improves the ability of thecontamination particles to stick to the walls defining the indirect gasflow path 46 of the particle trapping structure 52. For example, thewalls may be coated in an adhesive or the like. Also, in someembodiments the particle trapping structures may be heated so as toimprove the attachment of contamination particles within the gas to thewalls which define the indirect gas flow path 46 of the particletrapping structures 52.

Within the described embodiment, the membrane 44 is adjacent twoseparate particle trapping structures 52. It will be appreciated that alithographic apparatus according to the present invention may compriseonly one particle trapping structure or alternatively may comprise morethan two. Furthermore, any particle trapping structure may be located ata position remote to the membrane.

According to an alternative aspect, it is possible to provide alithographic apparatus that includes a radiation source, the radiationsource configured to produce a radiation beam, a support configured tosupport a patterning device, the patterning device being configured toimpart the radiation beam with a pattern to form a patterned radiationbeam, a chamber located between the radiation source and the support(and patterning device when the patterning device is being supported bythe support), the chamber containing at least one optical componentwhich is configured to reflect the radiation beam, the chamber beingconfigured to permit radiation from the radiation source to pass throughit to the patterning device and a membrane, the membrane beingconfigured to permit the passage of the radiation beam through themembrane, the membrane also being configured to prevent the passage ofcontamination particles through the membrane, the membrane comprising orbeing formed of graphene. It is not necessary that the membrane definespart of the chamber, nor is it necessary that the membrane lies in thepath of the radiation beam. In such an aspect, the membrane may form apellicle constructed and arranged to protect the patterning device fromparticles that form debris. It may also be used as a spectral purityfilter.

In the described embodiment the membrane and particle trapping structureare located within the illumination system IL at the end of the gasconduit 32 closest to the patterning device MA. This need not be thecase. For example, the membrane and particle trapping structure may bepositioned at the end of the gas conduit 32 which is closest to theintermediate focus IF. Additionally or alternatively, the membrane andparticle trapping structure may be located within the source module SO,for example at a position between the collector and the intermediatefocus IF. In a further alternative embodiment, the membrane may be suchthat it forms both part of the illumination system IL (the membranebeing intermediate optics within the illumination system and thepatterning device) and part of the projection system PS (the membranealso being intermediate optics within the projection system and thepatterning device). In this embodiment, the membrane separates thepatterning device from both the illumination system and the projectionsystem. The membrane may define, in part, a chamber within which thepatterning device is situated. It will be appreciated that in thisembodiment, the beam of radiation will pass through the membrane twice:once when it travels from the illumination system to the patterningdevice and once when it travels from the patterning device to theprojection system. The illumination system and projection system mayeach comprise a chamber which is defined in part by the membrane. Themembrane may be provided after the source module SO and before optics ofthe illumination system IL.

It will also be appreciated that a lithographic apparatus according thepresent invention may comprise more than one membrane.

A membrane formed from graphene may be provided at any suitable locationin the lithographic apparatus. The membrane may for example beconfigured to stop the passage of contamination particles through thelithographic apparatus.

As previously mentioned, a substance which may be suitable for use informing the membrane of the present invention may be graphene. Grapheneis commonly a planar sheet of carbon atoms in a honeycomb crystallattice. Graphene shows a high degree of transparency to extremeultraviolet radiation compared to other solid materials. Due to theseproperties graphene may also be used to form pellicles or spectralpurity filters.

Known pellicles which have been used with EUV radiation includepellicles made from a silicon/rubidium (Si/Ru) material. These materialshave been found to be undesirable due to the fact that they are fragile(i.e. not mechanically robust) and they have been found to suffer fromsubstantial transmission losses. Due to the lack of robustness ofpellicles made of Si/Ru materials, they are brittle and hence verydifficult to handle and locate within a lithographic apparatus.Furthermore, the lack of mechanical robustness of the Si/Ru material maymean that a pellicle formed of this material may not be able to maintainits integrity so as to prevent debris from reaching the reticle, or maynot be capable of withstanding the environmental factors within thelithographic apparatus. Such environmental factors may include pressuregradients within the lithographic apparatus and/or changes intemperature. As previously mentioned, Si/Ru materials have been found tosuffer from substantially large transmission losses. These transmissionlosses may be in excess of 50% of the incident EUV radiation. Anytransmission losses due to the pellicle will result in less radiationreaching a substrate within the lithographic apparatus. This may resultin a reduction of imaging performance of the lithographic apparatus.

FIG. 4 shows a graphene pellicle 64. The pellicle 64 is supported by apellicle frame 66 which holds the pellicle 64 at a fixed distance fromthe reticle 68. The reticle 68 lies in the focal plane of thelithographic apparatus and is an example of a patterning device.

The use of a pellicle is known as a way of preventing debris (e.g.contaminant or dust particles) from coming into contact with thereticle. Any debris which comes to rest on the reticle may causesubstantial degradation in the imaging performance of the lithographicapparatus because the reticle (and hence the debris in contact with thereticle) is in the focal plane of the lithographic apparatus. Aspreviously stated, the pellicle prevents debris from reaching thereticle. Any debris which comes to rest on the pellicle will not be inthe focal plane of the lithographic apparatus and therefore anydegradation in the imaging performance of the lithographic apparatuswill be much less than if the debris had come to rest on the reticle.

The use of graphene as a pellicle for use with EUV radiation may beparticularly advantageous. The pellicle 64 shown in FIG. 4 is a singlesheet of graphene. The sheet of graphene is mechanically robust, meaningthat it is capable of preventing debris particles from reaching thereticle 68. Furthermore, the graphene sheet pellicle 64 is thought to becapable of transmitting approximately 99% of EUV radiation incident uponit (both at 13.5 nanometres and at 6.7 nanometres). Due to the fact thatgraphene is formed from single atomic layers of carbon atoms, propertiesof the graphene are substantially uniform. For example, the thicknessand composition of the graphene sheet may be substantially uniformacross the entire sheet. This may be advantageous because the opticalproperties of the pellicle will be substantially the same for any partof the pellicle. This in turn means that all parts of any radiation beampassing through the pellicle will be affected by the pellicle in thesame manner. It follows that the pellicle will not affect the patterningof the radiation beam and hence the imaging performance of thelithographic apparatus of which the pellicle forms part may not bedetrimentally affected.

In an embodiment, instead of forming the pellicle 64 from a single sheetof graphene, the pellicle is formed from a plurality of graphene layerslocated on top of one another. This may provide the pellicle 64 withimproved strength and robustness. For example, more than ten graphenelayers or more than fifty graphene layers may be used to form thepellicle 64. A pellicle formed from a plurality of graphene layers mayhave a higher strength than a pellicle formed from a single sheet ofgraphene. Although the transmission of the pellicle may be reduced if itis formed from a plurality of layers, the pellicle may neverthelessstill have a sufficiently high transmission to allow it to be used in anEUV lithographic apparatus. The transmission of 100 layers of graphenemay be 85% at 13.5 nanometres and may be 95% at 6.7 nanometres. Thethickness of 100 layers of graphene is around 30 nanometres. Thepellicle 64 could be formed by stacking together single layers ofgraphene, or by epitaxially growing a stack of graphene layers.

The use of graphene to form the pellicle 64 has several advantages whichinclude a high mechanical strength, a high degree of uniformity (interms of both thickness and composition), a high transparency to EUVradiation, and a high degree of thermal stability (i.e. it issubstantially unaffected by changes in temperature which may occurwithin a lithographic apparatus). In addition, graphene is capable ofwithstanding temperatures of up to around 1500° C. Graphene's mechanicalstrength avoid the need to support the pellicle using a grid, and thusavoids the non-uniformity of EUV radiation that a grid would introduce.In an embodiment, a grid may be used to provide some support to thegraphene.

Graphene's mechanical strength means that the pellicle can be relativelyeasily handled. For example, a pellicle comprising graphene can bepositioned on a support member to which edges of the pellicle areattached, and may then be put in its desired location within thelithographic apparatus. The pellicle may be periodically replaced byremoving the support member and pellicle and replacing it with a newsupport member and pellicle. Graphene also has a substantially flatsurface which enables the graphene to be doped (if required) in auniform matter. Doping the graphene may alter certain properties of thegraphene as is well known in the art. In some pellicles, doping thegraphene may enable the spectral purity filters to exhibit desirableoptical and/or mechanical properties, for example greater radiationtransmission and/or greater mechanical strength.

FIG. 5 shows a second embodiment of pellicle 64. In this embodiment, thepellicle 64 is not made from a graphene sheet, but rather from anunordered arrangement of graphene flakes which form a layer thatconstitutes the pellicle 64. The graphene flakes may be of differentshapes and sizes (for example they may be up to approximately 100micrometers size). The graphene flakes are held together by Van derWaals' forces. A pellicle 64 formed from graphene flakes 70 may have aless uniform thickness and a less flat surface than a single sheet ofgraphene, and may be more reactive to hydrogen and oxygen in thelithographic apparatus because it includes more graphene edges. Anadvantage of forming the pellicle 64 made from graphene flakes 70 isthat it provides a similar performance to the graphene sheet but at areduced cost.

The pellicle 64 shown in FIG. 6 comprises two graphene sheets 72sandwiched between layers of support material 74 such that the pellicle64 is formed from a stack of alternating layers as follows: supportmaterial, graphene sheet, support material, graphene sheet, supportmaterial. Using the support material 74 in addition to graphene 72 mayprovide the advantage that it improves mechanical properties of thepellicle 64. The graphene may act as an anti-diffusion barrier.

It will be appreciated that the support material layers 74 may protectthe graphene sheet from other environmental factors within thelithographic apparatus, such as temperature and mechanical stress. Insome embodiments, it may be desirable that the layers of supportmaterial allow a significant amount of EUV radiation to pass throughthem. It will also be appreciated that although embodiment of thepellicle 64 shown in FIG. 6 comprises two graphene sheets sandwichedbetween the support material layers 74, any number of graphene sheets 72and support material layers 74 may be used. In addition, in the place ofthe graphene sheets 72, graphene flakes (as described above) may also beused.

It is conceivable that a pellicle may comprise a material layer ontowhich a layer of graphene or graphene flakes has been provided.

Graphene may also be used in the construction of spectral purityfilters. Known radiation sources within lithographic apparatus, such aslaser produced plasma (LPP) sources, may produce in-band radiation(which may be used to pattern the substrate) and out-of-band radiation.The in-band radiation may for example be extreme ultraviolet (EUV)radiation, whereas the out-of-band radiation may be infrared radiation.Infrared radiation has a wavelength in the range of approximately 0.8 toapproximately 1000 μm. For example, infrared radiation may have awavelength of approximately 10 μm. The out-of-band radiation may bereflected by the same reflectors which direct the in-band radiation tothe substrate. It may be undesirable that the out-of-band radiation isreflected by the lithographic apparatus onto the substrate because theout-of-band radiation may have a detrimental effect on the substrate.For example, in the case where the out-of-band radiation is infraredradiation, the infrared radiation may cause the substrate to be heated.Heating of the substrate may cause the substrate to expand. Expansion ofthe substrate may result in reduced imaging performance of thelithographic apparatus. For example, successive exposures of thesubstrate to the radiation beam may not align with one another. This maybe referred to as an overlay problem. Alternatively, in the case wherethe in-band radiation is EUV radiation, the out-of-band radiation may bedeep ultraviolet (DUV) radiation. DUV radiation may also cause areduction in imaging performance of the lithographic apparatus. Forexample the DUV radiation may cause a pattern formed imaged onto asubstrate to be blurred.

Spectral purity filters are a known way of suppressing the transmissionof out-of-band radiation (for example infrared and/or DUV radiation)through the lithographic apparatus to the substrate. Known spectralpurity filters suffer from the fact that the materials which are used toprevent the transmission of the out-of-band radiation (i.e. byabsorption and/or reflection) also prevent the transmission of usefulin-band EUV radiation. Because the amount of absorption of the in-bandradiation by the spectral purity filter increases with increasingthickness of the spectral purity filter, known spectral purity filtersfor use with EUV radiation are thin (50 to 100 nanometers thick) so asto minimize EUV radiation absorption. Due to the fact that thesespectral purity filters are so thin, they may be very fragile (i.e. theyhave a low mechanical strength). It is therefore very difficult tohandle and position such spectral purity filters within the lithographicapparatus. Furthermore, despite minimizing the thickness of knownspectral purity filters, the transmission of EUV radiation through aknown spectral purity filter may be less than about 50%. The lowtransmission of the EUV radiation through such spectral purity filtersmay be undesirable because a reduced intensity of EUV radiation reachingthe substrate may result in reduced imaging performance of thelithographic apparatus.

As previously discussed, graphene is a material which has a highmechanical strength and also permits substantial EUV radiation to betransmitted through it. FIG. 7 shows a spectral purity filter 76 whichincludes a graphene sheet 78. The graphene sheet 78 is disposed upon aspectral purity filter frame 80. The graphene sheet 78 supports anout-of-band radiation suppression structure 82. The out-of-bandradiation suppression structure 82 may comprise a single ormulti-layered structure which is capable of suppressing the transmissionof out-of-band radiation through the spectral purity filter. FIG. 7shows a radiation beam A which is incident on the spectral purityfilter. The incident radiation beam may comprise both in-band radiationand out-of-band radiation. The in-band radiation B passes through theout-of-band radiation suppression structure 82 and graphene sheet 78 andmay then pass to the patterning device and hence the substrate. Theout-of-band radiation may be absorbed by the out-of-band suppressionstructure 82 (absorption of out-of-band radiation not shown) or may bereflected away from the spectral purity filter 76 as indicated by C. Thegraphene sheet 78 acts to support the potentially thin out-of-bandradiation suppression structure 82 and therefore makes the spectralpurity filter easier to handle and locate within a lithographicapparatus. In addition, the graphene may also prevent oxidation of theout-of-band radiation suppression structure 82, since graphene isimpermeable to oxygen. Both sides of the out-of-band radiationsuppression structure 82 may be covered with graphene if it is desiredto prevent oxidation of the out-of-band radiation suppression structure.

FIG. 8 shows a similar spectral purity filter to that shown in FIG. 7.In this embodiment, the graphene sheet 78 of the spectral purity filtershown in FIG. 7 is replaced by graphene flakes 84. The graphene flakes84 may be in an unordered arrangement which forms a layer in whichdifferent shapes and sizes of flake are attached to one another withinthe layer via Van der Waals' forces. In some spectral purity filtersgraphene flakes may be advantageous compared to graphene sheets. This isbecause graphene flakes may be less expensive than graphene sheets, andmay be easier to process.

The spectral purity filter may for example comprise a grid which has apitch that is smaller than the wavelengths of radiation that it isintended to block. For example, the grid may have a pitch of around 5microns and may be used to block infrared radiation having a wavelengthgreater than around 10 microns. The grid may for example be formed fromhexagonal cells, e.g. having a honeycomb grid form. Ribs which form thehexagonal cells may for example be around 500 nanometres thick. The gridmay for example be formed from a metal such as tungsten which is capableof withstanding the substantial heat load generated by infraredradiation and which has a high emissivity. The outer surface of thetungsten grid will oxidize to form tungsten oxide when it is in theatmosphere, for example during construction of an EUV lithographicapparatus. When the EUV lithographic apparatus is operating, tungstenoxide will evaporate from the grid when it is exposed to infraredradiation. Some of the tungsten oxide may accumulate on optical surfacesof the EUV lithographic apparatus, decreasing their reflectivity andthereby decreasing the intensity of EUV radiation available forprojection onto a substrate. Conventional cleaning techniques are notcapable of removing tungsten oxide from the optical surfaces. It istherefore desirable to prevent tungsten oxide from the gridcontaminating the EUV lithographic apparatus.

Graphene may be used to protect the tungsten grid from oxidation and/orto prevent out-gassing of tungsten oxide from the grid. FIG. 9 showsschematically in cross-section an embodiment in which a spectral purityfilter 76 comprises a tungsten grid 77 (or grid formed from some othersuitable metal), a layer of graphene 79 being provided on either side ofthe grid. The graphene may be applied such that none of the tungstengrid 77 is exposed. The graphene layers 79 are not permeable to oxygen,and therefore will prevent oxidation of the tungsten grid 77. Thegraphene layers 79 may for example be applied as sheets of graphene tothe tungsten grid 77. This may be done when the tungsten grid is in avacuum (e.g. when a layer of tungsten oxide is not present on thetungsten grid). Once the graphene layers 79 are in place the spectralpurity filter 76 may be exposed to the atmosphere. The graphene layers79 prevent oxidation of the tungsten grid 77 and therefore prevent alayer of tungsten oxide from building up on the tungsten grid.

The graphene layers 79 may seal cells which comprise the grid (e.g.hexagonal cells in the case of a honeycomb structure). As a consequenceof this, the pressure within the cells may be significantly differentfrom the pressure outside of the cells. For example, there may be avacuum within the cells and atmospheric pressure outside of the cells.The graphene layers 79 will be sufficiently strong to withstand forcesarising from these pressure differences (a single atomic sheet ofgraphene can hold a pressure difference of vacuum to the atmosphere overa 5 micron pitch grid). The graphene layers 79 may bend inwards as aresult of the pressure difference, as is represented schematically inFIG. 9.

In an alternative embodiment, shown schematically in FIG. 10, instead ofproviding graphene layers on either side of a tungsten grid, graphene isused to surround individual ribs of the grid. The spectral purity filter76 comprises a grid 77 formed from tungsten (or some other suitablemetal), each rib of the grid being surrounded by graphene 81. Thegraphene 81 is applied such that none of the tungsten grid 77 is exposedto the atmosphere, and therefore prevents oxidation of the tungsten gridand prevents a layer of tungsten oxide from building up on the tungstengrid.

The embodiment shown in FIG. 10 may for example be made by sputteringgraphene flakes onto the grid 77. Alternatively, a sheet of graphene maybe placed on top of the grid 77 and then broken up by blowing air ontoit such that the graphene breaks up and adheres to ribs of the grid.

Tungsten grid spectral purity filters may have a limited lifetime. Aprimary reason for the limited lifetime is that tungsten grain formswhen the grid is at high temperatures (which will occur during operationof the lithographic apparatus). The tungsten grain causes the grid tobecome fragile such that it may eventually break.

In an embodiment, the formation of tungsten grain in a spectral purityfilter grid may be prevented or reduced by constructing the grid as atungsten/graphene multi-layered structure. The graphene in such amulti-layered structure acts as a barrier which prevents the formationof large tungsten grains. The graphene does not have a significanteffect upon properties of the tungsten such as the tungsten meltingtemperature. An example of a spectral purity filter 76 comprising a gridformed from ribs 77 which comprise a tungsten/graphene multi-layeredstructure is shown schematically in FIG. 11. The thickness of thetungsten layers of the spectral purity filter may be smaller than thewidth of the ribs 77 in order to limit the formation of tungsten grainssuch that they cannot have a size which exceeds the width of the ribs.For example, the ribs may be 500 nanometres thick. The tungsten layersmay be 100 nanometres thick or less, and may be 50 nanometres thick orless. The graphene layers may for example be less than 1 nanometrethick.

A multi-layered grid structure comprising graphene and some othersuitable material (e.g. some other suitable metal such as Rhenium) maybe used to form the spectral purity filter.

In an embodiment, instead of using a multi-layered structure to form thespectral purity filter grid, the grid may be formed from a mixture ofgraphene and tungsten. This may be achieved for example by mixinggraphene with tungsten and then sputtering the graphene and tungstentogether onto an optical component (e.g. an optical component of an EUVlithographic apparatus). Forming the spectral purity filter grid from amixture of graphene and tungsten may help to reduce the formation oftungsten grains. It may be desirable to use more tungsten than graphenein order to retain desirable properties of tungsten such as good heatresistance and high emissivity. The proportion of graphene to tungstenmay be relatively low, for example 5% or lower, and may be 1% or lower.

A mixture of graphene and some other suitable material (e.g. some othersuitable metal) may be used in an equivalent manner to form the spectralpurity filter.

In some instances, tungsten grains may become detached from the grid 77and may cause contamination in the EUV lithographic apparatus. Graphenemay be used to cover the grid 77 or surround constituent parts of thegrid, for example as shown in FIGS. 9 and/or 10. Where this is done thegraphene may act as a barrier which prevents tungsten grains fromcontaminating the EUV lithographic apparatus.

FIG. 12 shows an embodiment which comprises a spectral purity filterwith a multi-layered structure. This spectral purity filter comprisessix alternating layers: three graphene sheet layers 78 and threeout-of-band radiation suppression structure layers 82. This spectralpurity filter can therefore be likened to a stack of three spectralpurity filters which are shown in FIG. 7. This embodiment may beadvantageous compared with the embodiment shown in FIG. 7 because alarge total thickness of out-of-band radiation suppression layers 82 canbe supported by a plurality of graphene sheet layers 78. Due to the factthat multiple graphene sheet layers 78 are provided amongst theout-of-band radiation suppression structure layers 82, the spectralpurity filter may have a greater mechanical strength compared to aspectral purity filter with a single out-of-band radiation suppressionlayer and a single graphene sheet layer. It will be appreciated thatwithin this embodiment the graphene sheet layer 78 may be replaced withlayers made out of graphene flakes as described above.

An advantage of strengthening a spectral purity filter using graphene(e.g. in the manner shown in FIG. 12) is that it does not significantlychange the high emissivity of the spectral purity filter. Highemissivity of the spectral purity filter is useful because it allows thespectral purity filter to radiate heat efficiently and therefore allowsthe spectral purity filter to stay at a lower temperature duringoperation of the lithographic apparatus than would otherwise be thecase.

FIGS. 13 and 14 show further spectral purity filters which comprise asupport layer formed from graphene flakes 84. In both cases, the layerof graphene flakes 84 is disposed upon the out-of-band radiationsuppression structure 82. Disposed upon the layer formed from grapheneflakes 84 is a grid structure 86. The pitch of the grid may be chosen tosubstantially prevent transmission of out-of-band radiation (for exampleinfrared radiation) as explained further above. The pitch of the gridmay for example be 5 microns.

The grid 86 of the spectral purity filter shown in FIG. 13 comprises asupport structure 88 (made for example from tungsten) and a reflectivecoating 90 which is reflective to out-of-band radiation. An example of acoating which may be used in the case where the out-of-band radiation isinfrared radiation (for example at 10.6 μm) is molybdenum (Mo). Thereflective coating 90 may act together with the pitch of the grid toblock infrared radiation. grid may have dimensions to

The grid structure of the spectral purity filter shown in FIG. 11 ismade from a material which is reflective to the out-of-band radiation.In the case where the out-of-band radiation is infrared radiation (forexample at 10.6 μm), an example of a material which is reflective toout-of-band radiation is a metal with high conductivity. Examples ofsuch metals include Al, Au, Ag and Cu.

It will be appreciated that a spectral purity filter structure similarto that shown in FIG. 12 may be utilized in combination with a gridstructure which reflects out-of-band radiation (or which absorbsout-band radiation but does not transmit it). This may be useful forexample in an LPP radiation source, in which a substantial amount ofinfrared radiation may be present. A spectral purity filter similar tothat shown in FIG. 12 may be utilized without a grid structure an a DPPradiation source, where there may be little or no infrared radiation.For example, a grid structure which prevents transmission of out-of-bandradiation may be placed on top of the spectral purity filter shown inFIG. 12. One possible embodiment of such a spectral purity filtercomprises a grid with a spacing of 5 μm (the grid suppressingout-of-band infrared radiation having a wavelength of 10.6 μm). Thespectral purity filter also comprises three graphene layers, eachgraphene layer being a graphene sheet which is three layers of graphenethick (thickness of one layer of graphene is approximately 0.34 nm,therefore the thickness of a sheet of graphene which is three layersthick is approximately 1 nm). The spectral purity filter furthercomprises three out-of-band radiation suppression layers made fromsilicon, zirconium or molybdenum, each having a thickness ofapproximately 3 nm. In other embodiments the spectral purity filter maycomprise four or more (or two) graphene layers and out of bandsuppression layers. Each graphene layer may comprise four or more (ortwo) sheets of graphene.

Features of the spectral purity filter embodiments described above maybe combined. For example a tungsten grid in which constituent parts aresurrounded by graphene (e.g. as shown in FIG. 10) may be provided withan outer layer of molybdenum to provide increased reflectivity of out-ofband radiation.

It will be appreciated that the out-of-band radiation suppressionstructure may comprise any material which is capable of absorbing and/orreflecting out-of-band radiation whilst at the same time allowingtransmission of in-band radiation. Examples of materials which may beused include tungsten (W), silicon (Si) and zirconium (Zr).

Graphene may be used when constructing multi-layer mirrors, for examplemulti-layer mirrors used in EUV lithographic apparatus. Multi-layermirrors are formed from a plurality of alternating layers of a metalsuch as molybdenum or tungsten, and a spacer such as silicon, the layersbeing formed on a substrate which provides structural support. Thethicknesses of layer pairs may be equal to half the wavelength ofradiation to be reflected (e.g. EUV radiation at 13.5 nm). Constructiveinterference of radiation scattered from each layer pair causes themulti-layer mirror to reflect the radiation.

It is desirable in a multi-layer mirror to provide well definedboundaries between adjacent layers (e.g. between a layer of molybdenumand a layer of silicon). Over time diffusion may occur between adjacentlayers such that the boundaries between them become less well defined.The diffusion may in part be due to the mirrors being heated duringoperation of the lithographic apparatus. Graphene may be used as ananti-diffusion layer located between adjacent layers, the grapheneacting to maintain well defined boundaries between the adjacent layers.

Graphene is suited to use as an anti-diffusion layer because it isnon-permeable and may be provided in a thin layer (e.g. less than 1 nmthick) such that it absorbs very little EUV radiation. Because grapheneis able to withstand high temperatures (e.g. up to around 1500° C.) itmay allow multi-layer mirrors to withstand higher temperatures thanconventional multi-layer mirrors. Conventional multi-layer mirrors mayfor example become damaged at temperatures of around 100° C. If aspectral purity filter in an EUV lithographic apparatus were to fail,then infrared radiation incident upon multi-layer mirrors of thelithographic apparatus may quickly heat the multi-layer mirrors to morethan 100° C., causing damage to the multi-layer mirrors such that theymust be replaced. By using graphene to increase the maximum temperaturewhich multi-layer mirrors are able to withstand, the likelihood ofdamage to multi-layer mirrors due to overheating is reduced.

Using graphene to allow multi-layer mirrors to withstand highertemperatures than conventional multi-layer mirrors may allow multi-layermirrors to be used in locations in which they would conventionally notbe used. For example, multi-layer mirrors may be used before thespectral purity filter. For example a multilayer mirror (or mirrors) maybe used as a collector in the EUV radiation source SO.

An embodiment of a multi-layer mirror is shown schematically in FIG. 15.The multi-layer mirror comprises a substrate 92 upon which alternatinglayers of molybdenum 94 and silicon 96 (or other suitable materials) areprovided. A layer of graphene 98 is provided between each layer ofmolybdenum 94 and silicon 96. The layer of graphene may for example beless than 1 nm thick, or may have some other thickness.

The multi-layer mirror may for example be constructed by growing a layerof molybdenum or silicon onto the substrate (e.g. using chemical vapourdeposition), manually applying a sheet of graphene onto the layer,growing the next layer, etc.

It is conventional to provide a layer of ruthenium on the outermostlayer of a multi-layer mirror in order to prevent oxidation of the outerlayers of the multi-layer mirror. The ruthenium layer will oxidisespontaneously when it is exposed to the atmosphere, and will alsooxidise during operation of the EUV lithographic apparatus. Thisoxidation may reduce the reflectivity of the mirror by around 1%, whichmay be sufficient to give rise to a significant loss of EUV radiationintensity in the lithographic apparatus (the loss is cumulative for eachmirror of the apparatus). The oxidation can be removed from the mirrorby cleaning using hydrogen ions. However, a residue may be left behindon the mirror which cannot be removed during cleaning. The residue willbuild up on the mirror over time, thereby reducing the lifetime of themirror.

In an embodiment, a layer of graphene may be provided as the outermostlayer of the multi-layer mirror instead of the layer of ruthenium (orother capping layer). The graphene will prevent oxidation of the outerlayers of the multi-layer mirror because it is non-permeable for oxygen.

Using graphene in a multi-layer mirror (either between layers on top ofthe outermost layer) is advantageous because the graphene may beprovided with a uniform thickness (e.g. as a single layer) and may thusnot distort radiation reflected by the mirror. The same advantage mayapply to the graphene layers provided on either side of a spectralpurity filter (see FIG. 9).

Graphene provides the advantage that it may be provided as a very thinlayer (e.g. 0.34 nanometres for a single layer), in which case it ishighly transparent to EUV radiation (e.g. around 99.8% transparent for athickness of 0.34 nanometres). Furthermore, graphene is stable whenheated to temperatures which may occur in an EUV lithographic apparatus.A further advantage is that graphene is widely available and may beapplied over relatively large surface area.

Another advantage of graphene is that it is chemically resistant tohydrogen ions, therefore allowing cleaning within an EUV lithographicapparatus to be performed without the graphene being damaged. Graphenealso has the advantage, compared with Ruthenium, that tin and zinc donot adhere to it. When cleaning is performed using hydrogen ions, tinand zinc may tend to adhere to a Ruthenium outer layer of a mirror (thetin and zinc coming from other parts of the lithographic apparatus).This may form a residue which cannot easily be removed. This may limitthe lifetime of the mirror because it may become less reflective overtime. However, tin and zinc do not tend to stick to graphene. Thebuild-up of residue on the mirrors may therefore be avoided, and thelifetime of the mirror may be extended.

Although graphene is referred to above, graphene derivatives may be usedin mirrors, spectral purity filters, etc of an EUV lithographicapparatus. Graphene derivatives include for example graphane, graphenefluoride, graphene bromide, graphene chloride and graphene iodide.Graphene and graphene derivatives have in common that they are allmembranes which have carbon SP2 bonded bases. The mechanical propertiesof graphene derivatives may be the same or similar to the mechanicalproperties of graphene, although the chemical properties may bedifferent. Graphene fluoride may provide the advantage that it has bondswhich are less susceptible than graphene bonds to breaking whenilluminated by EUV radiation. For this reason, graphene fluoride may beused in embodiments of the invention instead of graphene.

Graphene may comprise a single layer of SP2 bonded carbon, or maycomprise a plurality of layers of SP2 bonded carbon stacked together, ora plurality of layers of predominantly SP2 bonded carbon stackedtogether.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

What is claimed is:
 1. A lithographic apparatus comprising: a radiationsource configured to produce a radiation beam; a support configured tosupport a patterning device, the patterning device being configured toimpart the radiation beam with a pattern to form a patterned radiationbeam; a chamber located between the radiation source and the support,the chamber containing at least one optical component configured toreflect the radiation beam, the chamber being configured to permitradiation from the radiation source to pass therethrough; a membranethat defines part of the chamber, the membrane configured to permit thepassage of the radiation beam through the membrane, and to prevent thepassage of contamination particles through the membrane; and a particletrapping structure configured to permit gas to flow along an indirectpath from inside the chamber to outside the chamber, the indirect pathof the particle trapping structure being configured to substantiallyprevent the passage of contamination particles from inside the chamberto outside the chamber, wherein the particle trapping structurecomprises first and second spaced wall members from each of which aplurality of plates extend, plates being spaced from one another, oneadjacent plate extending from the first wall member and the otheradjacent plate extending from the second wall member such that theplates form an interdigitated overlapping structure, and wherein themembrane is attached to the first wall member.
 2. A lithographicapparatus according to claim 1, wherein the indirect path is ameandering path.
 3. A lithographic apparatus according to claim 2,wherein the meandering path is boustrophedonic.
 4. A lithographicapparatus according to claim 1, wherein the plates are planar or curved.5. A lithographic apparatus according to claim 1, wherein the particletrapping structure comprises a sponge-like material configured to beimpermeable to contamination particles and to permit gas to flow throughit.
 6. A lithographic apparatus according to claim 1, wherein themembrane comprises at least one material selected from the groupconsisting of: zirconium, silicon, molybdenum, molybdenum disilicide,yttrium, rubidium, strontium, niobium, ruthenium, and graphene.
 7. Alithographic apparatus according to claim 6, wherein the membrane isformed from graphene or from a graphene derivative.
 8. A lithographicapparatus according to claim 1, wherein the membrane and particletrapping structure are part of a source collector module or anillumination system of the lithographic device.
 9. A lithographicapparatus comprising: a radiation source configured to produce aradiation beam; a support configured to support a patterning device, thepatterning device being configured to impart the radiation beam with apattern to form a patterned radiation beam; a chamber located betweenthe radiation source and the support, the chamber containing at leastone optical component configured to reflect the radiation beam, thechamber being configured to permit radiation from the radiation sourceto pass therethrough; a membrane that defines part of the chamber, themembrane configured to permit the passage of the radiation beam throughthe membrane, and to prevent the passage of contamination particlesthrough the membrane; and a particle trapping structure configured topermit gas to flow along an indirect path from inside the chamber tooutside the chamber, the indirect path of the particle trappingstructure being configured to substantially prevent the passage ofcontamination particles from inside the chamber to outside the chamber,wherein the particle trapping structure comprises first and secondspaced wall members from each of which a plurality of plates extend,adjacent plates being spaced from one another, one adjacent plateextending from the first wall member and the other adjacent plateextending from the second wall member such that the plates form aninterdigitated overlapping structure, and wherein the first and secondwall member each comprise an opening configured to allow the passage ofthe radiation beam.
 10. A lithographic method comprising: generating aradiation beam; directing the radiation beam through a chambercontaining at least one optical component that reflects the radiationbeam, the radiation beam being directed towards a patterning device, thechamber including a membrane; preventing the passage of contaminationparticles with the membrane when the radiation beam passes from thechamber the membrane and towards the patterning device; flowing gas frominside the chamber to outside the chamber along an indirect path througha particle trapping structure, the indirect path substantiallypreventing the passage of contamination particles from inside thechamber to outside the chamber; imparting the radiation beam with apattern to form a patterned radiation beam with the patterning device;and projecting the patterned beam of radiation onto a substrate with aprojection system, wherein the particle trapping structure comprisesfirst and second spaced wall members from each of which a plurality ofplates extend, adjacent plates being spaced from one another, oneadjacent plate extending from the first wall member and the otheradjacent plate extending from the second wall member such that theplates form an interdigitated overlapping structure, and wherein themembrane is attached to the first wall member.
 11. A method according toclaim 10, wherein the first and second wall member each comprise anopening configured to allow the passage of the radiation beam.
 12. Amethod according to claim 10, wherein the plates are planar or curved.13. A method according to claim 10, wherein the membrane comprises atleast one material selected from the group consisting of zirconium,silicon, molybdenum, molybdenum disilicide, yttrium, rubidium,strontium, niobium, ruthenium, and graphene or a graphene derivative.14. A method according to claim 13, wherein the membrane is formed fromgraphene or from a graphene derivative.
 15. A method according to claim10, wherein the membrane and particle trapping structure are part of asource collector module, or an illumination system, or positioned toseparate the patterning device from the projection system of alithographic apparatus.
 16. A lithographic method comprising: generatinga radiation beam; directing the radiation beam through a chambercontaining at least one optical component that reflects the radiationbeam, the radiation beam being directed towards a patterning device, thechamber including a membrane; preventing the passage of contaminationparticles with the membrane when the radiation beam passes from thechamber through the membrane and towards the patterning device; flowinggas from inside the chamber to outside the chamber along an indirectpath through structure, the indirect path substantially preventing thepassage of contamination particles from inside the chamber to outsidethe chamber; imparting the radiation bean with a pattern to form apatterned radiation beam with the patterning device; and projecting thepatterned beam of radiation onto a substrate with a projection system,wherein the particle trapping structure comprises a sponge-like materialwhich configured to be impermeable to contamination particles and topermit gas to flow through it.